Photovoltaic devices are known in the art (e.g., see U.S. Pat. Nos. 6,784,361, 6,288,325, 6,613,603, and 6,123,824, the disclosures of which are hereby incorporated herein by reference). Amorphous silicon photovoltaic devices, for example, include a front electrode or contact. Typically, the transparent front electrode is made of a pyrolytic transparent conductive oxide (TCO) such as zinc oxide or tin oxide formed on a substrate such as a glass substrate. In many instances, the transparent front electrode is formed of a single layer using a method of chemical pyrolysis where precursors are sprayed onto the glass substrate at approximately 400 to 600 degrees C. Typical pyrolitic fluorine-doped tin oxide TCOs as front electrodes may be about 400 nm thick, which provides for a sheet resistance (Rs) of about 15 ohms/square. To achieve high output power, a front electrode having a low sheet resistance and good ohm-contact to the cell top layer, and allowing maximum solar energy in certain desirable ranges into the absorbing semiconductor film, are desired.
It would be desirable to provide a technique and structure for improving the ability of the semiconductor film (or absorber) of the photovoltaic (PV) device to absorb light and thus generate electrical charges.
Certain example embodiments of this invention relate to a photovoltaic (PV) device including an electrode such as a front electrode/contact and a method of making the same. In certain example embodiments, the front electrode has a textured (e.g., etched) surface that faces the photovoltaic semiconductor film of the PV device. The textured surface of the front electrode, facing the semiconductor absorber film, is advantageous in that it increases the amount of incoming radiation or solar energy that is absorbed by the semiconductor film of the PV device. In certain example embodiments, the front electrode is formed on a flat or substantially flat (non-textured) surface of a front glass substrate, and after formation of the front electrode via sputtering or the like, the surface of the front electrode is textured (e.g., via etching). In completing manufacture of the PV device, the textured (e.g., etched) surface of the front electrode faces the active semiconductor film (or absorber) of the PV device.
The use of a front electrode having a textured surface adjacent the semiconductor film (or absorber) is advantageous in that it increases the optical path of incoming solar light within the semiconductor film through light scattering, thereby increasing the chance for photons to be absorbed in the semiconductor film to generate electrical charge.
In certain example embodiments, the front electrode is of or includes a transparent conductive oxide (TCO) film having first and second layers of the same or substantially the same material (e.g., zinc oxide, zinc aluminum oxide, indium-tin-oxide, or tin oxide). The first TCO layer of the front electrode is sputter-deposited using a ceramic sputtering target(s), and the second TCO layer of the front electrode, of the same material, is sputter-deposited using a metallic or substantially metallic sputtering target(s). This allows the better quality TCO of the film, deposited more slowly via the ceramic target(s), to be formed using the ceramic target and the lesser quality TCO of the front electrode film to be deposited more quickly and cost effectively via the metallic target(s). After the etching, most or all of the better quality ceramic-deposited TCO remains whereas much of the lesser quality metallic-deposited TCO of the film was removed during the etching process and is no longer present. This allows the TCO of the front electrode to be formed in a more efficient and cost effective manner, resulting in a better and more competitive product. In completing manufacture of the PV device, the etched surface of the front electrode faces the active semiconductor film of the PV device.
In certain example embodiments of this invention, the front electrode of a photovoltaic device is comprised of a multilayer coating including at least one conductive substantially metallic IR reflecting layer (e.g., based on silver, gold, or the like), and at least one transparent conductive oxide (TCO) layer (e.g., of or including a material such as zinc oxide or the like). In the PV device, the TCO is provided between the semiconductor film and the substantially metallic IR reflecting layer. The surface of the TCO layer may be etched to provide a textured or etched surface facing the semiconductor film. In other example embodiments, the front electrode may consist essentially of the TCO film having a first TCO layer and a second TCO layer of the same or substantially the same material, with a substantial part of the second TCO layer (but little or none of the first TCO layer) having been removed during the etching process.
In certain example embodiments of this invention, a multilayer front electrode coating may be designed to realize one or more of the following advantageous features: (a) reduced sheet resistance (Rs) and thus increased conductivity and improved overall photovoltaic module output power; (b) increased reflection of infrared (IR) radiation thereby reducing the operating temperature of the photovoltaic module so as to increase module output power; (c) reduced reflection and increased transmission of light in the region(s) of from about 450-1,000 nm, 450-700 nm and/or 450-600 nm which leads to increased photovoltaic module output power; (d) reduced total thickness of the front electrode coating which can reduce fabrication costs and/or time; (e) an improved or enlarged process window in forming the TCO layer(s) because of the reduced impact of the TCO's conductivity on the overall electric properties of the module given the presence of the highly conductive substantially metallic layer(s); and/or (f) increased optical path within the semiconductor film, due to the etched surface of the front electrode, through light scattering thereby increasing the chance for photons to be absorbed in the semiconductor film and through light trapping between the reflective metal back electrode(s) by multiple internal reflections so as to generate additional electrical charge.
In certain example embodiments of this invention, there is provided a method of making an electrode for a photovoltaic device, the method comprising: sputter-depositing a first transparent conductive oxide layer on a substrate using at least one ceramic sputtering target; sputter-depositing a second transparent conductive oxide layer using at least one metallic sputtering target to form a transparent conductive oxide film comprising the first and second transparent conductive oxide layers, wherein the second transparent conductive oxide layer directly contacts the first transparent conductive oxide layer and is of the same metal oxide material as the first transparent conductive oxide layer; and after said sputter-depositing steps, etching the transparent conductive oxide film to form a textured surface, wherein a substantial portion of the second transparent conductive oxide layer is removed during said etching and a majority of said first transparent conductive oxide layer remains after said etching.
In other example embodiments, there is provided a photovoltaic device comprising: a front glass substrate; a front electrode provided between the front glass substrate and a semiconductor film of the photovoltaic device, wherein the front electrode comprises a transparent conductive oxide (TCO) film comprising first and second TCO layers of the same TCO metal oxide material which directly contact each other, the first TCO layer being more dense than the second TCO layer of the same TCO material; wherein a major surface of the TCO film layer closest to a semiconductor film of the photovoltaic device is etched so as to be textured, and wherein the second TCO layer is closest to the semiconductor film and is discontinuous due to the etching.